The effect of boronizing on the microstructure and wear behavior of co-based Haynes 25 superalloy

Although cobalt and cobalt alloys have very high mechanical properties, their wear and corrosion resistance is not at a desired level for harsh service conditions, and therefore their application is limited in such cases. In order to minimize this negative effect, different surface treatments can be applied to cobalt alloys. Surface treatments are performed to improve the mechanical or tribological properties of materials such as hardness, ductility, fatigue, wear and corrosion. Boriding technique, which is among these processes, is a thermochemical coating method and is an important surface hardening process used in hardening the material surface. In this thesis, cobalt-based Haynes 25 superalloy, which is preferred in conditions where high wear and temperature resistance is required such as nuclear reactors, medical equipment and gas turbines, was used. 40x40x5 mm specimens were cut and molded, and surface cleaned gradually with 180-1200 SiC grinding papers and polished to remove dirt, rust and unwanted layers on the surface of the parts before proceeding to the surface hardening processes. As boronizing powder blend, 90% B4C (boron carbide) and 10% NaBF4 (sodium tetrafluoroborate) powders were used. Different boriding processes were applied to the prepared samples at 850, 950 and 1050°C temperatures for 4 hours. Sections were taken from the boronized samples and after metallographic preparation, their microstructures were examined under optical microscope and scanning electron microscope and layer thickness measurements were made. Optimization of boriding temperature and time was investigated with these measured values. In boronized samples, hardness measurements were made with a microhardness device in the boride layer, transition zone and base material regions, and the composition of the boride layer was revealed by X-ray diffractometry (XRD). Abrasion tests were applied to the boronized samples in a ball-on disc wear device with a 20 N load against the alumina (Al2O3) ball, over 250 m sliding distance, at room temperature and 500°C parameters. Thanks to the high hardness and self-lubricating properties of the boride layers obtained, a wear resistance of 18 times at room temperature and 4 times at 500°C was achieved compared to the untreated sample. The decrease in the improvement in the wear rates of the boride layers at high temperatures is due to the fact that the wear mechanism turns into fracture type wear besides oxidative wear at 500°C. On the other hand, while the wear mechanism of untreated Haynes 25 alloy occurred as mixed type (abrasive, oxidative and plastic deformation) at room temperature, it turned into oxidative and fracture type wear mechanism at 500°C.